1. Field of the Invention
The invention relates to the field of electronically steerable antennas and more particularly to electronically steerable antennas with optically coupled feed systems.
2. Description of the Prior Art
Conventional electronically scanned antennas are seldomly proposed for high gain, limited scan applications because they require an excessive number of control elements and phase shifters. To reduce the number of control elements and phase shifters, considerable attention has been devoted to limited scan antenna techniques. Conventional designs with narrowed field view produce only a limited savings on the number of phase shifters over wide angle antenna systems, because the maximum element spacings are constrained to avoid the formation of grating lobes. Several early efforts for achieving a limited scan capability with significant control elements and phase shifter reductions utilized a small electronically steerable array located in a focal region of a microwave optical system. These systems, however, exhibit low aperture efficiency, since only a portion of the aperture is illuminated for each scan angle.
Significant improvements in aperture efficiency and antenna component reductions were realized with the development of the overlapping subarray technique. This technique uses appropriate combinations of orthogonal beam formers and switching networks to achieve the desired scanning capability and beam characteristics. In these designs, the primary collimating device is a lens or reflector with subarraying networks, such as, Butler matrices or Rotman lenses located in the focal regions. Limited scan antennas of this type exhibit the unfavorable characteristics of a physically deep configuration which is associated with optically fed array systems.
Several alternatives have been proposed to the lens fed, sub-arraying systems. These alternatives fall into two general categories. The first, a conceptually straight forward extension of the lens fed system, simply substitutes a Butler matrix for the primary collimating lens. Because of the complexity of the Butler matrix this is not an attractive approach for large aperture antennas. The second uses partially overlapped or interlaced arrays. A variety of specific design approaches have been suggested which offer significant reduction in antenna depth. These designs, however, exhibit poor side lobe and loss performance with reduced scanning capabilities relative to the fully overlapped sub-arrays.
The present invention overcomes the physical drawbacks of the optically fed limited scan techniques while substantially achieving performance characteristics of fully overlapped sub-arrays.